Polyethylene films have been widely used as packaging materials but, in most cases, they assume the form of high-pressure-processed low-density polyethylene (LDPE) and low-pressure-processed high-density polyethylene (HDPE). Recently, there has been available L-LDPE (low-pressure-processed linear low-density polyethylene) called the third polyethylene and having the advantages of both low- and high-pressure-processed polyethylenes. Now, attention is paid to the possibility of using such L-LDPE as low-cost and high-strength polyethylene to meet current demand for energy and resources savings. However, L-LDPE is of transparency unsuitable for use in general packaging, so that it cannot be substituted for the prior art high-pressure-processed low-density polyethylene. In addition, it has been considered difficult to use L-LDPE even in a packaging field for which no transparency is needed owing to its unsatisfactory processability. In other words, L-LDPE, i.e., a copolymer of ethylene with other alpha-olefin, does not only need an extra amount of energy for the formation of films, but often leads to the necessity of new installations or modifications of film-making machines.
Packaging materials for the complete shielding of light are used to package light-sensitive materials, viz., photosensitive materials which lose their quality upon exposure to light. The properties then demanded include gas-shielding properties, light-shielding properties, moistureproofness, various physical strength (breaking strength, tear strength, impact perforation strength or extent of impact perforation, Gelbo test strength, etc.), heat sealability (heat seal strength, hot sealability, etc.), antistatic properties and the like. It is difficult for a single film material to possess all the aforesaid properties. For that reason, use has generally been made of composite laminated films comprising polyethylene films blended therein with carbon black, pigments, etc., and flexible sheets formed of, e.g., paper, aluminum foils, cellophane or the like.
Nonetheless, the prior art films have still had the disadvantages that their physical properties are not sufficient, i.e., they are torn or perforated during packaging, they peel off at the heat sealed portions, etc. Furthermore, the addition of a light-shielding substance such as carbon black results in a lowering of physical strength. It has been required, therefore, that the amount of the light-shielding substance to be added be limited to about 3%, while the films have a thickness exceeding 70 microns. This leads to bulky packaging, increased rigidity of the films, deterioration of packaging workability, and rises in the cost.
As the representative packaging materials for photosensitive materials there have been available laminates wherein two layers, a flexible sheet layer and a carbon black-containing polyethylene (LDPE) film layer are laminated on both sides of an aluminum foil as shown in FIG. 3. The reason for using the aluminum foil is to eliminate the disadvantages that the thermoplastic resin film is poor in moistureproofness, likely to be electrically charged or to be peeled-off, etc. The results are, on the contrary, that tear strength, extent of impact perforation and Gelbo test strength deteriorate considerably, as compared with the sole use of thermoplastic resin films and, in particular, a breakage problem arises in the packaging of heavy articles. In consequence, the thickness of the carbon black-containing LDPE films has been fixed at 70 microns or more.
As one manner for solving the problem that the addition of a light-shielding substance to the conventional high-pressure-processed low-density polyethylene (LDPE) films or the low-pressure-processed high-density polyethylene (HDPE) films leads to drops of the physical properties thereof, the present inventors have already proposed in U.S. Pat. No. 4,452,846 corresponding to Japanese Patent Kokai Publication No. 58-132555 a light-shielding film comprising an L-LDPE polyethylenic polymer and no less than 1% by weight of a light-shielding substance, wherein a low-pressure-processed linear low-density polyethylene (L-LDPE) accounts for no less than 50% by weight of the overall polyethylenic polymer. When composite laminated films (C3 and C4 in Table 1) have been made by simply using this prior art film and an aluminum foil without any modification, it has been found that, though they are more excellent than the conventional composite laminated films (R2, R3, R4) using carbon black-containing LDPE light shielding films and an aluminum foil, however, as compared with the light-shielding film (C2) based on L-LDPE alone of the prior application, their tear strength reduces to a half or less in spite of an increase in their thickness. Thus, it has been impossible to considerably reduce the thickness of such laminated films.